Organic light emitting diode (OLED) devices are increasing becoming the display of choice for a wide range of applications. For example, OLED devices are increasingly being used as displays for computers, laptops, personal digital assistance and cellular phones, just to name a few of their ubiquitous applications. Following their example in liquid crystal display technology, there are two main system architectures for OLED displaysxe2x80x94passive and active matrix displays. For high resolution passive matrix OLED displays, one row is addressed at a time. For example, in an OLED display with M rows and an average luminance of L, the pixels in the same row will be driven to a peak brightness of M*L. For a 1000 line display, the peak brightness could exceed 200,000 nits and the voltage required to drive the OLED pixels could exceed 20V. Thus, the passive matrix OLED device may become very inefficient and the display power consumption high.
In order to reduce the power consumption of an OLED display, an active matrix scheme may be highly desirable. In this case, every pixel typically has a switch, a memory cell and a power source. When a row of pixels is addressed, the pixel switch is turned on and data is transferred from the display drivers to the pixel memory capacitors. The charge is held in the capacitor until the row is addressed in the next frame cycle. Once the charge is stored in the capacitor, it turns on the power source to drive an OLED pixel and the pixel will remain on until the next address frame cycle.
As a device, an OLED is commonly characterized as a xe2x80x9ccurrent devicexe2x80x9dxe2x80x94as its light output is proportional to its current input. To achieve good control of the luminance uniformity and good control of gray scale across the entire display, a current source is typically used to drive the OLED device. Therefore, the power source used in an active matrix OLED is usually a current source.
One such current source architecturexe2x80x94as is known in the field of active matrix OLED display (AMOLED)xe2x80x94is shown in FIG. 1. The basic scheme in the field of OLED displays is a two transistor circuit with one transistor being a switch for the data and the other one being a current source. FIG. 1 depicts a typical thin film transistor 100 as is known in the art. The data line is connected to the drain (104) of transistor T1 (102) is connected and the select line is connected to the gate (106). The source of T1 is connected to a capacitor CS (108) and to the gate of transistor T2 (110). The drain of T2 112 is connected to Power and the source of T2 is connected to the pixel area 114.
In operation, T1 is the switching transistor that allows data charges to be stored in the storage capacitor 108. The stored charge in the storage capacitor 108 turns on the current source transistor T2 110. The drain of the current source transistors T2 supplies the current to the pixel 114 whereby the brightness of the pixel is determined by the drain current in the transistor T2. The drain current (ID) of the transistor T2 is controlled by the charge stored at the storage capacitor 108.
FIG. 2 shows the operating characteristics of transistor T2 as a plot of ID versus VDS. A family of curves are shownxe2x80x94with each curve depicting operation at a different VGS. As can be seen, dotted line 202 broadly defines two separate operating regions of transistor T2xe2x80x94the xe2x80x9clinear regionxe2x80x9d 204 and the xe2x80x9csaturation regionxe2x80x9d 206, as is well known in the art. To operate transistor T2 as a current source, it is typical to select a VGS1 in the saturation region of transistor T2. Once selected, the current is fairly constant and is independent of the value of VDS1. To control the luminosity of the pixel, it is again typical to select the VGS. As can be seen, with higher values of VGS, the greater the amount of ID flows through the pixel and, hence, increases its light output.
In constructing the circuit of FIG. 1, thin film transistors (TFTs) are typically used to fabricate the pixel power source because of their relatively low cost. TFTs are widely used in AMLCD today in most high resolution flat panel displays. Most of the TFT""s used today for AMLCD are made with amorphous silicon (a-Si) because of the low manufacturing cost. However, a-Si TFT has inherently low carrier mobility (xcx9c1 cm2/V-s) and the transistor size is relatively large. This limits the resolution of the displays fabricated with a-Si as well as the capability of using it as a current source.
For displays with fine pitch, polycrystalline Si (p-Si) is used for TFT fabrication because the size of the TFTs can significantly reduced. Typically, the electron mobility in p-Si is close to 100 cm2/V-s while the hole mobility is about 50 cm2/V-s. Since current source is used to drive AMOLED displays (and, in particular, those employing OLED pixels), p-Si typically chosen for TFT fabrication because of the high current capability of p-Si. However, there are many issues associated with using p-Si for TFT fabricationsxe2x80x94and particularly when used in OLED displays.
For example, since current sources are commonly used to drive the pixel, the current source TFTs need to have a high current capability. Even with p-Si, the transistor size has to be fairly large relative to the pixel size, resulting in low pixel fill factor. As a result, pixels have to be driven at a higher pixel brightness and this reduces the panel power efficiency and device lifetime. In addition to the cost disparity between a-Si and p-Si TFTs, it is desirable to use a-Si for the driver circuitry of an active matrix display.
Second, the pixel power consumption is then equal to I*(VPIXEL+VDS), where VDS is the source-drain terminal voltage across the TFT and VPIXEL is the voltage across the cathode and the anode of the pixel. As noted above, for current-source operation, a TFT is usually operated in its saturation region. Under this operation, VDS can be quite large, typically in the range of 5-7 V for p-Si. On the other hand, VPIXEL is only about 3 V (in particular, for OLED pixels). As a result, over 60% pixel power consumption is due to the TFT circuitry. Thus, it is highly desirable to reduce the power consumption of the TFT circuitry.
Additionally, there is a problem using TFTs for a current source. The current in the TFT current source is determined by the difference between VGS and the threshold voltage of the gate terminal, VT. The threshold voltages in p-Si TFT are typically non-uniform across the display. This non-uniformity has a big impact on the TFT drain current. Typically, IDxcx9c(VGSxe2x88x92VT)2; thus, a small variation in VT could have a big change in ID. Several alternative approaches have been proposed to use a more complex circuitry (3-5 TFTs) to compensate for the drift in the threshold voltage. This approach increases the process complexity and affects yield. Since more transistors per pixel are used in the display, it further decreases the pixel fill factor, resulting in a display with lower efficiency and poor lifetime.
One embodiment of the present invention recites a driver circuit for an active matrix display, said driver circuit comprising:
a first transistor, said first transistor comprising a source, a drain and a gate;
a storage capacitor, said storage capacitor comprising a terminal, said terminal connected to one line, said one line comprised of a group of said source and said drain of said first transistor;
a second transistor, said second transistor comprising a source, a drain and gate, wherein said gate is connected to said terminal of said storage transistor;
wherein said drain and said source of said second transistor are connected to one of group, said group comprising a power source and a pixel element respectively; and
further wherein storage capacitor is chargeable to sufficiently high voltage to operate said second transistor in its linear region of operation.